Power-line carrier communications are communications using a power line for supplying commercial power as a transmission line and usually are conducted by balanced transmission using two lines. In indoor power-line communications, communication machines are connected through a modem to a power supply outlet to which two or three power wires are connected and transmission and reception are executed between the communication machines connected to the outlet.
The indoor power line is not essentially laid as a communication line and thus is not necessarily be placed in an environment suited to communications. For example, if an electric machine (any other electric machine than a communication apparatus for conducting power-line carrier communications) is connected to an outlet, the input impedance of the electric machine is generally small as a high frequency and thus a so-called short stub state is entered and reflection of a high frequency signal occurs in the portion of the outlet and a communication signal is degraded.
Thus, an art of connecting a high-impedance element as a high frequency such as an inductance element between a power line and the power supply passage of an electric machine and resolving degradation of a communication signal is proposed (for example, refer to patent documents 1 and 2). This art makes it possible to suppress the effect of the impedance of an electric machine on power-line carrier communications and also suppress noise occurring from the electric machine.
However, in the arts in patent documents 1 and 2, an inductance element, etc., is connected directly or through a power plug connected to a power outlet, etc., between a power jack to which a power plug of an electric machine and a power line and thus the power outlet to which the inductance element, etc., is connected is used only for electric machine connection and if a communication apparatus is connected to such a power jack, a communication signal is attenuated by the inductance element, etc., and the state is not suited to communications. Therefore, preferably a communication apparatus is not connected to a power outlet, etc., to which the inductance element, etc., is connected.
Patent document 3 describes a power line relay including an output terminal for communication apparatus connection and an output terminal for electric machine connection other than a communication apparatus, and patent document 4 describes a power outlet apparatus including an outlet part for communication apparatus connection and an outlet part for electric machine connection other than a communication apparatus. Such a power line repeater or a power outlet apparatus makes it possible to connect both a communication apparatus and an electric machine other than a communication apparatus.
However, if a power plug of a machine is erroneously inserted, namely, if an electric machine other than a communication apparatus is connected to the output terminal or the outlet part for a communication apparatus, the effect on power-line carrier communications cannot be suppressed and if a communication apparatus is connected to the output terminal or the outlet part for an electric machine other than a communication apparatus, attenuation of a communication signal occurs and it becomes impossible to conduct efficient power-line carrier communications.
Patent document 1: JP-A-2003-283390
Patent document 2: JP-A-2003-188778
Patent document 3: JP-A-2004-304365 (FIG. 7)
Patent document 4: JP-A-10-65583 (FIG. 5)